Laser illumination arrangement for a cathode ray tube

ABSTRACT

A photosensitive layer of or for a cathode ray tube is illuminated by means of a laser illumination arrangement. The laser illumination arrangement comprises a homogenizer which serves to reduce the coherence of the laser beam.

BACKGROUND OF THE INVENTION

The invention relates to a laser illumination arrangement forilluminating a photosensitive layer in the course of the manufacture ofa cathode ray tube, said laser illumination arrangement comprising alaser and an optical imaging arrangement for imaging laser light ontothe photosensitive layer which is applied to a display window of or fora cathode ray tube.

Such illumination arrangements can be used to manufacture a cathode raytube, inter alia, for creating phosphor patterns and black matrixpatterns on a display window of a cathode ray tube.

An illumination arrangement of the type mentioned in the openingparagraph is disclosed in U.S. Pat. No. 4,117,177. In this patentspecification, a description is given of an arrangement in which aphotosensitive layer is illuminated by means of a laser beam. Thephotosensitive layer is applied to a surface of a display window of orfor a cathode ray tube. Illumination takes place by imaging laser lightonto and scanning it across the photosensitive layer. A shadow maskcomprising a large number of openings is situated between theillumination arrangement and the photosensitive layer. The laser lightis directed through the openings in the shadow mask at such an anglethat a pattern is formed behind the shadow mask. This process isrepeated a number of times, at a number of different angles, therebycreating a number of patterns. In this manner, phosphor patterns and/orblack matrix patterns can be provided.

Although the use of a laser for illuminating the photosensitive layeroffers a number of advantages, it has been found in practice that byusing the above-described method, the illumination pattern is subject tovariations, leading to an increase of the number of rejects (=thenumber/percentage of illuminated photosensitive layers which do not meetthe quality requirements imposed and hence must be removed from theproduction process).

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved illuminationarrangement of the type mentioned in the opening paragraph.

To achieve this, a laser illumination arrangement in accordance with theinvention is characterized in that the arrangement comprises a beamhomogenizer between the laser and the optical imaging arrangement or asa part of the optical imaging arrangement.

A beam homogenizer reduces the coherence of a laser beam incident on thebeam homogenizer.

A laser beam is a concentrated coherent beam of light. In practice ithas been found that undesirable interference patterns and reflectionpatterns occur. The beam of coherent laser light interferes with itself,thereby causing interference patterns. Also scattering at, for example,dust particles in the air or on an optical element, such as a lens or amirror, occurs. These interference patterns and scattering patternscannot be predicted. Outside and superposed on the actual beam, thepatterns form a kind of fleck pattern. As a result, light of a deviatingintensity, i.e. of an intensity other than the desired intensity, isincident on the photosensitive layer at locations where this isundesirable. This leads to rejects. In the laser illuminationarrangement in accordance with the invention, the laser beam iscontrolled by a beam homogenizer. In said beam homogenizer, the coherentlaser beam which is incident on the beam homogenizer is converted to amuch less coherent beam. In fact, a plurality of sub-laser beams areformed in the beam integrator, which jointly form a broad beam. Sincethe coherence is reduced substantially, or preferably has disappeared,interference patterns of the laser beam at itself occur to a muchsmaller degree. Reflection at dust particles may still occur, but sincethe beam has generally become broader, the intensity of reflected lightwill be smaller, and more importantly, since the coherence of the laserbeam is reduced, the reflected light will generally be spread over amuch larger space angle (and will thus be much less concentrated). Themaximum intensity deviation which can occur at a spot of thephotosensitive layer as a result of reflection at dust or otherparticles has been reduced by orders of magnitude.

Unlike the known state of the art, the laser beam is preferably notscanned across the photosensitive layer, but instead, the laserillumination arrangement comprises a wide-angle objective for imagingthe laser beam onto the photosensitive layer. This has the advantagethat moving parts are not necessary. Moving parts cause vibrations whichmay disturb the image or the setting of the laser illuminationarrangement. In addition, moving parts are susceptible to failure andrequire relatively much maintenance work.

The use of a wide-angle objective enables a virtual light source to becreated. Preferably the arrangement is such that the size of the virtuallight source ranges between 1 and 2.5 mm. If the light source is smallerthan 1 mm, the intensity of the light exhibits pronounced ripples behindan opening in the shadow mask, which is undesirable. If the size exceeds2.5 mm, the intensity becomes lower. Preferably, the size exceeds 1.5mm.

Preferably, the beam homogenizer is formed such that the shape of thebeam issuing from the beam homogenizer is substantially identical tothat of the photosensitive layer of or for a cathode ray tube. For thisreason, for illuminating a 4×3 display screen, use is made of a laserbeam (after having been guided through the beam homogenizer) with alength/width ratio of approximately 4×3; for a 16×9 display screen, useis made of a laser beam having a length/width ratio of 16×9 etc. Byvirtue thereof, the illumination efficiency (the amount of light used)can be increased.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 schematically shows a laser illumination arrangement inaccordance with the prior-art arrangement;

FIG. 2 schematically shows an example of a laser illuminationarrangement in accordance with the invention;

FIG. 3 schematically shows another example of a laser illuminationarrangement in accordance with the invention;

FIG. 4 is a schematic, sectional view of a beam homogenizer, in thisexample a beam integrator;

FIG. 5 schematically shows a wide-angle objective which can suitably beused in a laser illumination arrangement in accordance with theinvention;

FIG. 6 graphically shows the connection between the size of the virtualsource and the shape of the intensity of the light behind a shadow mask.

The Figures are diagrammatic and not drawn to scale. In the Figures,like reference numerals generally refer to like parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic and simplified view of a laser illuminationarrangement as shown in U.S. Pat. No. 4,177,177. The laser illuminationarrangement comprises a laser 1 and an arrangement 2. The arrangement 2includes a first set 3 with lenses L1 and L2, a scanning portion 4 witha set of movable mirrors M1 and M2 which can be rotated about axes M1and M2, a second set 5 with lenses L3 and L4, and a further scanningportion 6 with a movable mirror M3 which can be moved about the axes Cand D. The sets of lenses cause the beam diameter to be enlarged and thebeam 7 to be imaged onto a photosensitive layer 9 on display window 10.A shadow mask 8 is situated in front of the photosensitive layer. Thescanning portions deflect the beam and scan the photosensitive layer.

FIG. 2 is a schematic, simplified view of a laser illuminationarrangement in accordance with the invention. In this example, thephotosensitive layer is scanned. A beam homogenizer is arranged betweenthe laser 1 and the lens L1. In this beam integrator, the diameter ofthe laser beam incident on an entrance plane is increased and the laserbeam is homogenized, that is to say that the intensity of the laserlight emerging from the exit plane is approximately equal throughout thesurface of the exit window, and that the coherence of the emergent lightexhibits a reduction, or has disappeared. In this example, the beamhomogenizer is arranged between the laser and the lens L1. This is notto be interpreted in a limiting sense. The beam homogenizer may, forexample, also be arranged between the lens L2 and the mirror M1.

FIG. 3 shows a further example of a laser illumination arrangement inaccordance with the invention. A laser beam 7 generated by the laser 1(laser 1 is not shown in FIG. 3) passes a telescope lens system L1, L2.Subsequently, the direction of the laser beam 7 is changed 90 degrees byprism P1, the beam is homogenized in the beam homogenizer 21, the beamdirection is changed 90 degrees by prism P2 and the beam imaged is ontoa photosensitive layer 9 on the display window by a wide-angle objective32. The beam homogenizer may include, for example, an elongated (havinga length of, for example, 25 to 50 cm) quartz rod. As a result ofinternal reflections at the side walls of the rod, the coherence of theentering laser beam 7 is reduced, and preferably disappears. Twosub-beams 7 a and 7 b of the laser beam are shown as well as their beampaths through the wide-angle objective 32. It seems that all sub-beamsoriginate from a virtual source (VP) in the wide-angle objective. The(virtual) size of the virtual source is a parameter which may beimportant, as will be explained hereinbelow. In this example, thearrangement further includes a Fresnel lens 33 having a large number offacets. This lens causes the imaging of the laser beam to be improved,in particular the angle at which the laser beam is incident on thephotosensitive layer. During illumination, the Fresnel lens may performa small movement to preclude that the transitions between the facets arevisible on the photosensitive layer. In front of the entrance side ofthe rod 21 there is arranged, in a preferred embodiment, a diffuser 34.By means of a diffuser the laser beam is allowed to pass in a diffusedmanner and the angle of dispersion of the laser beam is increased. As aresult, more reflections occur at the side walls of the rod, thecoherence of the laser beam is further reduced and the homogeneity ofthe laser beam (in terms of intensity) across the exit window 35 isincreased. Preferably, the exit window is imaged onto the photosensitivelayer 9 so as to be slightly out of focus. In the case of sharpfocusing, a dust particle on the exit window may cause an undesirablereduction in intensity on the photosensitive layer. By imaging out offocus, this problem is reduced substantially.

FIG. 4 is a schematic, sectional view of a further example of a beamhomogenizer. This type of beam homogenizer may be used as an alternativeto the rod shown in FIG. 3. In this example, the beam homogenizercomprises an entrance side 22 having a large number (for example 32(4×8)) lenses 23. These lenses form a number of sub-beams which arefocused in a plane 27. This plane is imaged in the F-point (see FIG. 3).Said entrance side further includes a large number of lenses 24 and alens 25. The laser beam incident on the lenses 23 is divided into anumber of sub-beams, three of which (28, 29 and 30) are shown in FIG. 4.Small path-length differences occur between the sub-beams. Therefore,the emergent beam, all sub-beams of which converge in plane 31, iscomposed of a large number of sub-beams. This results in a reduction ofthe coherence of the light. The plane 31 is imaged onto thephotosensitive layer 9 (see FIG. 3). In this manner, the coherence ofthe light is reduced. The plane 31 is imaged onto the photosensitivelayer 9 (see FIG. 3). In this example, this plane may be imaged so as tobe sharply focused because it is not formed by a fixed surface (as inthe case of the rod shown in FIG. 3).

FIG. 5 is a schematic, sectional view of a wide-angle objective 32. Thelaser beam is imaged by the wide-angle objective 32 onto thephotosensitive layer 9 via openings in the shadow mask 8. All lightbeams seem to originate from a virtual source VP. Therefore, in thisarrangement the photosensitive layer is illuminated and exposed in asingle run. Unlike the known arrangement, the laser beam is not scannedacross the photosensitive layer. The advantage of the use of awide-angle objective relative to an arrangement as shown in FIG. 1 inwhich the laser beam is scanned across the photosensitive layer, is thatno moving parts are required. Moving parts cause vibrations which mayadversely affect the illumination and/or the arrangement. Besides,scanning may cause so-called stitching problems. The laser beam isscanned across the photosensitive layer in a number of zones. Problemsmay arise at locations where the zones demonstrate an overlap. If theparts to be illuminated do not demonstrate an overlap, then a portion ofthe photosensitive layer cannot be illuminated, however, if they dodemonstrate an overlap then a portion of the photosensitive layer isilluminated twice. In either case, problems may occur. Non-illuminatedparts are useless, while twice-illuminated parts have received too muchlight. In the arrangement shown in FIG. 3, these problems do not occurbecause the photosensitive layer is integrally illuminated by one laserbeam. It is noted that if the above-mentioned problems (interferencephenomena) do constitute a problem in an arrangement wherein the laserbeam is scanned, these problems are greater if scanning does not takeplace. The problems caused by said interference patterns are slightlyreduced by scanning if a laser beam is scanned. Unexpected andundesirable differences in intensity are distributed over a largesurface area by the movement of the beam, so that the effect thereof issmaller. This “smoothing” effect does not occur in an illuminationarrangement in which the laser beam is stationary.

As mentioned hereinabove, the dimension of the virtual source VP isimportant if illumination takes place via a shadow mask.

FIG. 6 shows the intensity distribution on the photosensitive layerbehind an opening in the shadow mask as a function of the diameter ofthe virtual source VP (ΦVP). At a diameter of 0.5 mm, the intensityclearly exhibits a peak. This is undesirable. This peak disappears asthe diameter of the virtual source increases. This increase of thediameter causes the maximum intensity to decrease. For this reason,preferably, the dimension of the virtual source (that is the diameter inone direction) ranges between 1 and 2.5 mm.

Hereinabove, the invention has been described and illustrated by meansof a laser illumination arrangement. Hereinbelow, a description will begiven of the method in accordance with the invention:

In the manufacture of a cathode ray tube, in which, in a process step, aphotosensitive layer on a display window of or for a cathode ray tube isilluminated by light from a laser beam, the laser beam is guided, beforereaching the photosensitive layer, through a beam homogenizer. As aresult, the occurrence of disturbing interference patterns is reduced,or prevented. Preferably, the laser beam is further guided through awide-angle objective, so that the laser beam is spread over thephotosensitive layer. Further preferred embodiments of the invention inaccordance with the invention are formed, inter alia, in that the laserlight is guided through a diffuser.

It will be obvious that, within the scope of the invention, manyvariations are possible.

What is claimed is:
 1. A laser illumination arrangement for illuminatinga photosensitive layer in the course of the manufacture of a cathode raytube, said laser illumination arrangement comprising a laser and anoptical imaging arrangement for imaging laser light onto thephotosensitive layer which is applied to a display window of or for acathode ray tube, characterized in that the laser illuminationarrangement comprises a beam homogenizer between the laser and theoptical imaging arrangement or as a part of the optical imagingarrangement.
 2. A laser illumination arrangement as claimed in claim 1,characterized in that the beam homogenizer is formed such that the laserbeam emerging from the beam homogenizer is rectangular in shape.
 3. Alaser illumination arrangement as claimed in claim 1, characterized inthat the laser illumination arrangement includes a wide-angle objectivefor focusing light emerging from the beam homogenizer.
 4. A laserillumination arrangement as claimed in claim 3, characterized in thatthe laser illumination arrangement is such that it seems that, viewedfrom the photosensitive layer, the laser beam originating from thewide-angle objective comes from a virtual source, which virtual sourcehas a dimension, in one direction, ranging between 1 and 2.5 mm.
 5. Alaser illumination arrangement as claimed in claim 1, characterized inthat the laser illumination arrangement comprises means for scanning thephotosensitive layer with the laser beam.
 6. A laser illuminationarrangement as claimed in claim 1, characterized in that the laserillumination arrangement comprises a diffuser arranged in front of thebeam homogenizer.